Siméon Denis Poisson

Siméon Denis Poisson was born in 1781 in Pithiviers, south of Paris, and rose through the École Polytechnique to become one of the great French mathematical physicists of the early nineteenth century. Laplace and Lagrange spotted his talent as a student; he was appointed to the faculty at nineteen and remained one of the leading figures at Polytechnique for the rest of his life.

Poisson's work ranged across mechanics, elasticity, heat, electricity, and probability. In mechanics, he reformulated Lagrange's methods in ways that clarified the role of the momentum conjugate to a coordinate, and introduced the brackets that now bear his name — the direct classical ancestor of the quantum commutator. His Traité de Mécanique (1811, revised 1833) laid angular momentum on a rigorous vector footing and codified precisely how torque changes it, the statement that would become τ = dL/dt.

He was a careful, prodigious author: over three hundred memoirs during his life, including the Poisson distribution (1837), Poisson's equation for gravitational and electrostatic potentials, and the analysis of elastic bodies that introduced the quantity now called Poisson's ratio.